The present invention relates to a method for determining base length, for coordinate measurements by forward triangular intersection, using at least two angle-measuring devices (theodolites) at a fixed distance apart.
A method of measuring large industrial objects is described in "Bild der Wissenschaften", Volume 8, 1981, at page 14. In that method, two theodolites with electronic data output are connected to a common microcomputer which calculates in absolute value the coordinates x, y, z of sighted object points, from observed values for the horizontal angle .gamma..sub.i and the vertical angle .epsilon..sub.i, as measured by the respective instruments. The basis for the calculations is the distance apart of the two measuring instruments, known as the base length, which should be ascertained as accurately as possible, since this base length determines the scale of the object coordinate system.
Direct measurement of the base length is frequently not possible, or it is possible only at excessively great expense. As a rule, the base length, therefore, is determined by sighting various check points on the object to be measured; then, from the known distances apart or from the coordinates of said points, the base length is determined by reversal of the computational procedure which is to be carried out in making the actual measurement.
It is clear that, with such a determination of base length, the error in measurement is greatly dependent on the geometry of the object and that it is of the same order of magnitude as for obtaining the actual measured values of the object coordinates.
However, a more accurate determination of the base length is desirable, approximately of the same order of magnitude as the precision of measurement of the angle-measuring instruments used.
To determine distances spaced apart in the range between about 2 meters and 20 meters, a so-called "subtense bar" is frequently employed in geodetics. This is a horizontal bar of 2 meters length; at each of its two ends a two-dimensional target mark is provided, at very precisely known spacing. When suitable material is selected for the bar, it is also stable with changes in temperature. Such bars are described, inter alia, in German Gebrauchsmuster patents Nos. 1,766,274 and 7,021,136.
If it is desired to use these bars for a determination of base length, they must be aligned very accurately and, after the sighting by one instrument, they must be reversed and adjusted in rotation, for sighting by the second instrument. In this connection, however, a zero-point error occurs since the axis of rotation generally does not precisely intersect the line connecting the two target marks.